MediaPlayer: some shortcuts tweaks

* F: toggle fullscreen (like tab)
* V: removed. Stopping the replay would lose the current position, and
we don't want non-undoable lossy operation on single keystrokes
* X: toggles play/pause instead of forcing play (like space)
* Alt+Shift+0/1/2/3 changes the aspecdt ratio to "raw", "default",
"4/3", "16/9".

Fixes remaining parts of #2495.